Unless you're running a REALLY tight small track with virtually no straight I'm pretty sure you'll find that 60mm is very low for a 13.5 (I've run more than that with 10.5). You have correctly calculated that your 52mm tire gets you RIGHT at 60mm with zero room for tire wear but the handling will always be a problem with tires that large PLUS you will be buying new tires left and right because you'll be chunking the crap out of them. Between buying new tires every week and all the farting around with this and that to TRY and get larger pinions wedged in there you'll spend WAY more than you would for a half dozen appropriate spur gears (even 88T would probably get you into the zone) shipped in from someplace.

Heck, if you bought one of my 3.2R's (bit more room for gearing, lowered pods for more use out of tires, thicker chassis, etc) and included money for however many spur gears you wanted I'd order them in and drop 'em in the box to ship with no extra postage. I'm sure others would perform similar service if there is some other car you'd prefer. The sad fact is the the L4 was getting a bit long in the tooth even BEFORE the brushless revolution. I'm ALL for keeping costs in check, etc, and getting more use out of "veteran" equipment but at some point you have to stop and really think if you're really getting any benefit from all the effort and money expended or if you're just beating your head on a wall. Or decide you're just happy participating and running your car around the track but don't want to be at the pointy end of the field and that's cool too. What you'll find is that someone WILL gear their 13.5 appropriately high and you'll be left in the dust.